If only you knew what you’ve been driving over for the last few years.

From the underbelly of the decrepit Pulaski Skyway, state Transportation Commissioner Jim Simpson pointed up at the bottom of the famed bridge Monday.

Hidden from the commuters who traveled above was a cross between Swiss cheese and steel. Holes rotted right through the I-beams — the result of a potent mix of water, rust, rock salt and neglect.

"This bridge is a D-minus ready to become an F," Simpson said.

The gaps were found after engineers removed the concrete encasement about a year ago.

On Saturday, the Skyway is being closed northbound toward New York City for two years while it undergoes a $1 billion rehabilitation. Two southbound lanes from New York to New Jersey will remain open during the project.

Simpson gave reporters a down-under tour of the steel-truss bridge Monday, inviting them to join him a couple at a time on his bucket truck to inspect the results of 82 years of wear-and-tear.

If the bridge rehabilitation were delayed further, he said, "we could lose this bridge and it could get to the point where we’d have to just shut it down one day, and we’d look at it as a tourist attraction — we wouldn’t allow cars on it anymore. That’s how critical it is to get this job done now."

"This condition can’t stand anymore," he said. "We have no more opportunity for Band-Aids."

Simpson assured that the 3½-mile-long bridge between Newark and Jersey City is safe to drive over at the moment because temporary supports — vertical and horizontal beams — are bolstering it.

"Our riders are going to be on a brand-new roadway with new steel floor beams, new stringers (and) a much more improved 21st century road deck," he said.

Alternatives to the northbound Pulaski Skyway include the eastbound New Jersey Turnpike extension in Hudson County — where the shoulder has opened as a third lane during rush hours — and Truck Routes 1&9.

Among other alternatives, NJ Transit has added rush hour trains on the Morris and Essex and North Jersey Coast Lines, as well as extra train cars on the Raritan Valley Line.

Still, Richard Hammer, a DOT assistant commissioner who oversees the capital program, is expecting "three weeks of trouble" while commuters try to learn the new traffic patterns and travel alternatives.

"We know it’s going to be bad at the beginning, there’s no doubt about it," he said. "Nobody’s expecting this to be a panacea on Day One. But we believe, like water, it’s going to have an equilibrium. It’s going to balance out over time. So we’re confident within a few weeks, things should settle down somewhat."

The two-year project will include a complete rehabilitation of the bridge deck and replacement of deteriorating beams.

Engineers considered keeping open the Pulaski Skyway to one lane of traffic in each direction, but determined the bridge was too narrow. They also considered alternating the direction of travel based on the morning and evening rushes, but said stopping traffic and moving barriers would have been problematic.

So they decided to keep the Skyway open only for the two southbound lanes from New York to New Jersey during the construction. After two lanes are rehabilitated, the traffic will shift to those two lanes while the others are being rehabilitated — but the direction of travel will always be south during the construction project, officials said.

Hammer said the bridge, New Jersey’s largest, was designed by railroad engineers and well-built at the time.

"It is an iconic structure. It’s special to New Jersey," he said. "I think it means something to a lot of people in New Jersey, and our job is to make sure that this thing is here for another 80 years or more, and that it’s safe."